The present disclosure relates to an image forming apparatus transferring a toner image formed on an image carrier onto a recording sheet in an electrostatic transferring manner.
In a case where an electrophotographic image forming apparatus, such as a copying machine or a printer, is left for a long time under a high humidity environment, such as in the summertime or in the rain, without driving, because a toner and a recording sheet are humidified, decrease in image density and disturbance of an image easily occur at a first JOB after starting the driving particularly. In particular, when the toner is humidified, it is feared that a toner charging amount is decreased to deteriorate the image density.
In order to prevent decrease in image density under the high humidity environment, a development device may carry out toner ejecting control or idling control. In the toner ejecting control, the toner in the development device is ejected on an image carrier and the ejected toner is then corrected by a cleaning part. In the idling control of the development device, a developing roller or the like of the development device is idled without forming an electrostatic latent image on the image carrier. Combination of the toner ejecting control and the idling control is particularly effective. However, in the toner ejecting control, a toner amount consumed other than image forming is increased. In the idling control, a user is kept waiting in the idling operation. Thus, either control manner is not a preferable manner for the user.
Thereupon, in an electrostatic transferring device electrostatically transferring the toner image formed on the image carrier onto the recording sheet, in order to adapt for the decreased toner charging amount, a countermeasure applying a transferring bias higher than an ordinary transferring bias is considered.
Alternatively, there is an image forming apparatus configured to control a transferring bias on the basis of a stopping period from finishing of one image forming operation to starting of next image forming operation and temperature near a fixing device.
However, in case of applying the high transferring bias, if an applying time is prolonged, because an electric charge with a reversed polarity to a charged polarity of an image carrier is accumulated inside the image carrier, it is feared that a static eliminator cannot eliminate the electric charge or the image carrier is frictionally worn or damaged.
In a case of controlling the transferring bias on the basis of temperature condition of the fixing device, because humidification condition of the toner is not taken into account, it is impossible to improve decrease in image density under the high humidity environment.